AirAware › Questions answered
Short, sourced answers to the questions people in Singapore ask most about the PSI, AQI, PM2.5 and haze. Each answer starts with the essentials, then links to NEA, MOH or US EPA sources. AirAware is independent and does not give medical advice.
The Pollutant Standards Index is Singapore’s official air-quality index, published by NEA for five regions. It covers six pollutants (PM2.5, PM10, sulphur dioxide, nitrogen dioxide, ozone and carbon monoxide); NEA calculates a sub-index for each and reports the highest as the PSI.
MOH’s health advisory is based on the 24-hr PSI, which NEA recalculates every hour over a rolling 24 hours. See the live PSI by region. NEA FAQ.
A 24-hr PSI from 101 to 200 is in NEA’s Unhealthy band. MOH advises healthy persons to reduce prolonged or strenuous outdoor physical exertion; the elderly, pregnant women and children to minimise it; and people with chronic lung or heart disease to avoid it.
From 201 to 300 (Very Unhealthy) the advice tightens, and above 300 (Hazardous) everyone should minimise or avoid outdoor activity. See the full MOH tables. HealthHub.
The PSI is Singapore’s official index; the AQI is the US EPA’s. Both run from 0 to 500 but use different breakpoints, and NEA notes the PSI referenced the US system without being identical to it. Since 2024 the same air can read worse on the US AQI than on the PSI.
AirAware’s AQI figure is its own PM2.5-only estimate, not an official Singapore index. How it is calculated. NEA FAQ · US EPA.
PM2.5 is fine particles 2.5 micrometres across or smaller, the main pollutant in haze smoke, small enough to reach deep into the lungs. It is measured as a concentration in µg/m³, not as an index.
NEA groups 1-hr PM2.5 into four bands: Normal (0–55), Elevated (56–150), High (151–250) and Very High (251+). Live PM2.5 by region. NEA.
They often measure or calculate different things. NEA says indices on other websites, such as aqicn.org, use other methods and are not comparable with the 24-hr PSI. Common reasons are different scales and breakpoints, different averaging periods (1-hour, NowCast or 24-hour) and different instruments.
The US EPA notes that only a few low-cost sensors estimate particulate matter within 30% of reference monitors. Sites can also differ in which stations they use and when they last updated. AirAware shows NEA’s readings with their observation times. NEA FAQ · AirNow · US EPA.
NEA publishes new readings every hour for the five regions. The 24-hr PSI is recalculated each hour over a rolling 24-hour period, so it moves slowly even when the air changes quickly. AirAware checks for new readings every five minutes and shows each reading’s observation time.
The 1-hr reading reflects the past hour and shows the current air; MOH suggests using it for activities in the next hour. The 24-hr reading averages the past day and feeds the 24-hr PSI, which is for planning ahead.
If 1-hr PM2.5 stayed in the Elevated band (56–150) for a full day, NEA says the 24-hr PSI would be roughly in the Unhealthy range. MOH · NEA FAQ.
Check the 1-hr PM2.5 for the next hour. Under MOH’s guide: Normal (0–55), carry on; Elevated (56–150), reduce strenuous outdoor activity; High (151–250), avoid strenuous outdoor activity; Very High (251+), minimise all outdoor activity. Vulnerable persons should be stricter at each level.
If you feel unwell, seek medical advice. Activity-by-activity guide. MOH · HealthHub.
MOH’s advice is stricter for the elderly, pregnant women, children and people with chronic lung or heart disease. N95 masks are not needed for short exposures such as commuting, or indoors. HealthHub advises an N95 when the PSI forecast is above 300 for healthy people, or above 200 for people with chronic lung or heart disease.
Anyone with breathing difficulty should consult a doctor before using an N95, and anyone feeling unwell should seek medical advice. HealthHub · MOH.
NEA says the transboundary haze affecting Singapore mainly comes from forest and peat fires elsewhere in the region. It is most likely during the Southwest Monsoon (June to September, according to MSS), when winds blow from the southeast to the south and can carry smoke here.
ASMC monitors regional fires and hotspots, and Singapore’s Transboundary Haze Pollution Act 2014 addresses conduct that causes haze pollution here. Today’s haze readings. NEA · MSS · ASMC.
Sources: NEA air pollution FAQs · haze.gov.sg (NEA) · HealthHub: protect yourself against haze · US EPA: PM NAAQS and AQI fact sheet (2024) · ASEAN Specialised Meteorological Centre